Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normal to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer or film on the receiving surface of the substrate.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by way of soldering, brazing, diffusion bonding, clamping, epoxy cements, or with interlocking annular members. High purity metal and metal alloy sputter targets have also been mechanically bonded to backing plates, for example as disclosed in U.S. Pat. Nos. 6,749,103 B1 and 6,725,522 B1.
The relatively high yield strength coefficient associated with copper and copper alloy or aluminum alloy backing plates, compounded by the use of higher levels of sputtering power required to energize larger sputtering targets, has increased the material stresses imposed on the bonds that join the sputter targets to the backing plates. Under such sputtering conditions, the sputtering target assemblies tend to deflect or separate upon exposure to the changing temperatures that are typically encountered during sputtering. To a certain extent, soft solders have accommodated stresses exerted on the sputter target/backing plate assemblies as the assemblies are heated during the sputtering process and subsequently cooled. When low strength backing plates have been employed in the target/backing plate assemblies, the assemblies tend to deflect significantly during sputtering causing several undesirable effects, such as additional particle generation, decreased uniformity, etc.
Accordingly, there remains a need in the art for a method for bonding sputtering target materials to an alloy backing plate wherein the assembly exhibits significant strength and yield strength. Although sputtering target assemblies may be made using high strength backing plate materials such as Al—SiC composite material (U.S. Pat. No. 6,183,686) or Al alloy 7075, such materials are often impossible to weld at circumferential surfaces due to their chemical composition and poor weldability. In some other cases, there is a need to join an Al alloy target to a Cu alloy backing plate, which is difficult without applying high temperature bonding processes, such as a diffusion bonding process. As is known in the art, such high temperature processes may result in undesirable Al alloy target recrystallization or weak bond strength due to formation of brittle intermetallic compounds between the metals (in this case Cu and Al).